1. Technical Field
The present invention relates to a light-emitting device including an organic EL (electroluminescent) element and the like, a method for driving the same, and an electronic device.
2. Related Art
An OLED (organic light-emitting diode), or in other words, an organic EL element, is a thin and lightweight light emission source. Organic EL elements are structured so that at least one layer of organic thin film containing an organic compound is interposed between a pixel electrode and a counter electrode. The pixel electrode functions as, for example, an anode, while the counter electrode functions as a cathode. When an electric current flows between the pixel electrode and the counter electrode, electrons and electron holes recombine in the organic thin film, causing the organic thin film and organic EL element to emit light.
JP-A-2007-310311 (hereinafter referred to as Patent Document 1), JP-A-2008-191296 (hereinafter referred to as Patent Document 2), and JP-A-2008-256916 (hereinafter referred to as Patent Document 3) disclose examples of such organic EL elements and image display devices provided therewith.
Organic EL elements such as that described above are driven by a driving circuit that has an appropriate structure. Such a driving circuit is a circuit that supplies, to the organic EL element, the source-drain current of a driving transistor based on the gate potential thereof. In this case, the emission luminescence of the organic EL element can be adjusted and so on by adjusting the gate potential.
However, such driving circuits have various issues that need to be solved. As an example of such an issue, there is variance in various properties of driving transistors, such as the mobilities or threshold voltages thereof. An image display device such as that mentioned earlier is typically provided with multiple organic EL elements and a driving circuit including driving transistors associated with each of the organic elements. However, if the properties of the multiple driving transistors vary due to variance in various parameters during the manufacturing process and the like, variance will also arise in the adjustment and so on of the emission luminance of the organic EL elements. This interferes with improvements in the quality of the displayed image.
The aforementioned Patent Documents 1 to 3 disclose techniques for handling such a problem. While Patent Document 1 focuses upon “threshold voltage” and “mobility” ([0004] in Patent Document 1), Patent Document 2 considers primarily a “case where the optimal mobility correction time is short”, and attempts to provide a technique for “suppressing variance in the luminance by making the variance in correction time relatively small” through the extension of the correction time, the correction time variance being caused by variance in the pulse widths of the input signal voltage write pulse ([0017], [0018] in Patent Document 2).
Patent Document 3 attempts to provide a technique for optimizing a mobility correction process “dependent on a video signal (driving signal, luminance signal) Vsig” ([0015] to [0017] of Patent Document 3).
According to these documents, certain effects can be achieved with respect to the aforementioned mobility variance. However, further issues still remain even if such mobility compensation (“correction” in the aforementioned documents) is carried out. For example, there is a problem that can be called the “tone dependence” of mobility compensation. That is to say, when performing mobility compensation according to a set time and a set procedure, there are cases where the mobility compensation operations can effectively suppress variance in the emission luminance when the organic EL element emits light of a certain specific tone, but may not suppress such variance when emitting light of other tones.
Furthermore, because restrictions may be placed on the amount of current supplied to organic EL elements in mobility compensation operations, there is the possibility that the operations cause difficulties in the realization of a desired emission luminance, and in particular, in the realization of a higher emission luminance.
More specifically, the aforementioned Patent Documents 1 to 3 disclose the techniques described hereinafter. Patent Document 1 discloses a technique for handling mobility variance by “adding correction for the mobility of a driving transistor to a signal voltage” for an “appropriate” amount of time in the gate of the driving transistor (claim 2 of Patent Document 1), in addition to correcting the threshold voltage (claim 1 of Patent Document 1). Patent Document 2 discloses a technique for “increasing the voltage value of the input signal voltage in stages” in order to so-called “precharge” the driving transistor in an early “stage”, thereby reducing the gate-source voltage of that driving transistor and “increasing the optimum mobility correction time” (claim 1 and [0021] of Patent Document 2).
Although these techniques may be capable of solving the issues the aforementioned Patent Documents 1 and 2 focus upon, these techniques are not capable of effectively handling the aforementioned problems such as mobility correction tone dependence, restrictions on emission luminance, and so on.
Furthermore, Patent Document 3 discloses a technique for reducing “the influence of lows and highs in a video signal Vsig on the mobility correction process” by applying a “correction voltage”, of “a value dependent on the video signal and lower than the video signal”, to the gate of the driving transistor (claims 1 and [0035] of Patent Document 3).
Patent Document 3 may be relevant to the aforementioned mobility correction tone dependence problem, because a “correction voltage” “dependent on the video signal” is used (for example, see [0098] and on in Patent Document 3). However, in Patent Document 3, the “correction voltage” is applied using the time within what is known as the write interval of the video signal, and a light emission control transistor (“TEL—C” in Patent Document 3) that controls the conductive state between the source potential and the organic EL element being in an ON state at that time is a prerequisite (see, for example, FIG. 3, (C) in FIG. 6, and [0077] and on in Patent Document 3; see also the passage “ . . . mobility correction process . . . that applies a voltage from a current supply unit to the source-drain area of one of the driving transistors” in claim 1). It can thus be said that this technique deals entirely with the gate-source voltage of the driving transistor that is to be achieved during mobility correction, or to put it differently, how the effectiveness within the concept of the so-called “mobility correction process” can be improved. Therefore, the technique of Patent Document 3, too, does not necessarily suitably solve the various problems “after” the aforementioned mobility correction.